Linear olefins are one of the most useful classes of hydrocarbons used as raw materials in the petrochemical industry and among these the linear alpha-olefins--unbranched olefins whose double bond is located at a terminus of the chain--form an important subclass. Linear alpha-olefins can be converted to linear primary alcohols by hydroformylation (oxo synthesis); alcohols of carbon number less than eleven are used in the synthesis of plasticizers whereas those of carbon number greater than eleven are used in the synthesis of detergents. Hydroformylation also can be used to prepare aldehydes as the major products which in turn can be oxidized to afford synthetic fatty adds, especially those with an odd carbon number, useful in the production of lubricants. Linear alpha-olefins also are used in the most important class of detergents for domestic use, namely the linear alkylbenzenesulfonates, which are prepared by Friedel-Crafts reaction of benzene with linear olefins followed by sulfonation.
Another important utilization of alpha-olefins is radical hydrobromination to give primary bromoalkanes which are important intermediates in the production of thiols, amines, amine oxides, and ammonium compounds. Direct sulfonation of the alpha-olefins afford the alpha-olefin sulfonates, a mixture of isomeric alkenesulfonic adds and alkanesulfones, which are effective laundry agents even in hard water and at low concentrations. Linear alpha-olefins, particularly those of eight carbons and under also are used as comonomers in the production of high density polyethylene and linear low density polyethylene.
Although linear olefins are the product of dehydrogenation of linear alkanes, the major portion of such products consists of the internal olefins. Preparation of alpha-olefins is based largely on oligomerization of ethylene, which has as a corollary that the alpha-olefins produced have an even number of carbon atoms. Oligomerization processes for ethylene are based mainly on organoaluminum compounds or transition metals as catalyst. Using catalytic quantities of, for example, triethylaluminum, the oligomerization of ethylene proceeds at temperatures under 200.degree. C. to afford a mixture of alpha-olefins whose carbon number follows a Schultz-Flory distribution. In the C6-C10 range there is less than 4% branched alpha-olefins, but the degree of branching increases to about 8% as the chain length is extended to 18. A modified process, the so-called Ethyl process, affords a high conversion of ethylene to alpha-olefins with a more controlled distribution but product quality suffers dramatically, particularly in the content of branched olefins. Thus, in the C14-C16 range linear alpha-olefins represent only about 76% of the product.
A notable advance in the art accompanied the use of transition metals as catalysts for ethylene oligomerization. The use of, for example, nickel, cobalt, titanium, or zirconium catalysts afforded virtually 100% monoolefins with greater than 97% as alpha-olefins, under 2.5% as branched olefins, and under 2.5% as internal olefins. Since the catalysts are insoluble in hydrocarbons, oligomerization by catalyst systems based on transition metals typically is performed in a polar solvent to solubilize the catalyst. Ethylene and its oligomers have limited solubility in the polar solvents used, consequently the oligomerization process is associated with a 3-phase system; a polar liquid solvent phase containing the catalyst, a second liquid hydrocarbon phase (consisting of the oligomers produced), immiscible with the polar liquid phase, and ethylene in the vapor phase. Such a system permits of a continuous oligomerization process, since ethylene can be introduced into the polar phase and oligomerization products can be withdrawn as the hydrocarbon phase.
Ethylene oligomerization affords alpha-olefins with a Schultz-Flory distribution which is catalyst dependent and, at least for the catalysts of major interest herein, temperature dependent to only a minor degree. Murray recently has described a class of catalysts having a transition metal component particularly attractive as oligomerization catalysts; U.S. Pat. No. 4,689,437, U.S. Pat. No. 4,716,138, and U.S. Pat. No. 4,822,915. See also U.S. Pat. No. 4,668,823. Using such catalysts under conditions where the Schultz-Flory distribution constant is about 0.65 affords an oligomerization product whose alpha-olefin distribution in the C6-C16 range is particularly desirable from an economic viewpoint. That is, the economic value of ethylene oligomers may be maximized by having a Schultz-Flory distribution of about 0.65. A concomitant of oligomerization at such conditions is the production of about 10% of oligomers having 20 or more carbon atoms (C20+) which are solids at ambient temperature, and therein lies the problem whose solution is our invention.
The C20+ oligomers have limited solubility in the hydrocarbon phase of the oligomerization process described above, hence tend to separate as waxy solids. The oligomerization process then becomes a four-phase system; a vapor phase of ethylene, a polar solvent phase with dissolved catalyst, an immiscible liquid hydrocarbon phase, and a solid phase (wax) of C20+. The formation of solids tends to plug the reactor as currently configured, so a continuous process becomes interrupted periodically due to the necessity of unplugging the reactor, and even during process operation liquid flow is impeded as solids build. We have solved this problem by recycling a portion of the lighter oligomers, viz., the C12-C18 oligomers, to the reactor in order to solubilize the C20+ formed and avoid plugging. In outline form our process is the formation of ethylene oligomers with the desired Schultz-Flory distribution, especially one near 0.65, separating and recovering the C6-C10 oligomers, separating the C20+ solids, separating and recovering the C12-C18 oligomers substantially free of C20+, and recycling a portion of the C12-C18 fraction sufficient to maintain a homogeneous hydrocarbon phase in the reactor. Thus, the recycled C12-C18 acts as a solvent for the C20+ oligomers.
Although the foregoing is an important variant it is only representative of a much larger class. In general, oligomerization of ethylene governed by a Schulz-Flory distribution will give heavy oligomers, i.e., oligomers of sufficiently high carbon number that they are waxy solids only partly soluble in the oligomeric product mix under oligomerization process conditions. These solids will be a problem in causing reactor plugging, and prevention of solid separation is highly desirable. This can be effected by increasing the solubility of the heavy oligomers in the liquid hydrocarbon phase by recycling some of the lighter oligomer fractions to the hydrocarbon phase, with the exact composition of the recycled phase quite dependent upon the carbon number of the heavy oligomers separating as a waxy solid, the oligomerization process temperature, the amount of recycle which can be tolerated or which is desirable, the nature of the fractionating process, and so forth. But in all events a lighter fraction is recycled to solubilize the heavy oligomers and thereby avoid plugging. The case described in the foregoing paragraph then becomes a specific embodiment of our more general invention.